Polymeric materials based on acrylic monomers are known in the art, including those whose primary use is for pressure sensitive adhesives (for example, U.S. Reissue No. 24,906). PSAs are typically copolymers of a major proportion of alkyl esters of acrylic acid and a minor portion of at least one modifying monomer, such as (meth)acrylic acid, (meth)acrylamide, (meth)acrylonitrile and the like. Acrylate-based polymers are widely used as adhesives industry for reasons of costs, raw material availability, ease of reaction and properties.
However, some acrylate monomers are highly volatile and require expensive equipment. For example, methyl acrylate is a highly volatile monomer and using such a monomer in an adhesive formulation requires the use of coating equipment that is certified as Class 1, Division 2, Group D for use with flammable volatile materials as designated by the US National Electric Code. Coaters of this type tend to be very expensive and hazardous to operate. A number of polymerization methods have been used, but few, if any deal adequately with the problems of highly volatile monomers.
It is known in the literature and in the industry that there are at least five different feasible methods for the production of acrylic-based pressure-sensitive adhesives (hereinafter "PSAs"). These known methods include solution polymerization, emulsion polymerization, suspension polymerization, irradiation by high energy particulate matter (for example, electron beams or gamma rays), and ultraviolet light (hereinafter "UV") photopolymerization. As explained below, there are disadvantages and/or limitations incurred with the use of each known process.
Solution polymerization is used because it is relatively easy to control the significant reaction exotherm characteristically associated with acrylate polymerization. However, elaborate drying ovens with massive exhaust ducts and high temperatures are required to carry away the volatile organic compounds (hereinafter "VOCs") after coating from solution. Furthermore, to prevent the VOCs from being vented to the atmosphere (with resulting pollution and solvent loss), expensive VOC recovery equipment is necessary. Safety hazards in such operations are also significant, as the VOCs are extremely flammable and precautions must be taken to avoid explosive mixtures in the oven and recovery systems. Further, coatings applied from solution have a limitation as to the thickness of the coating that can be deposited in one pass through the coater. Thus, with coatings above about 0.125 mm, multiple coating layers must be deposited in successive trips through the coater to avoid blistering of the coating due to solvent evaporation.
While emulsion and suspension polymerizations have minimized the problems associated with the handling and evaporation of flammable solvents, heat must be supplied to remove water from the coating and essentially the same equipment must be employed. Though high solids coatings are possible, the higher heat of vaporization of water as compared to VOCs offsets this benefit and about the same total energy for drying is required. Drying times are relatively long, thus limiting production rate. One of the most serious limitations of the emulsion polymerization process is the water sensitivity of the resulting polymers (caused by the emulsifying agent, which is carried along in the process and becomes part of the final adhesive). A further limitation of this process is that highly polar monomers, which are water miscible, are difficult to incorporate into the copolymer during polymerization and considerable homopolymerization of such monomers can occur in the aqueous phase.
More recently, development work has been done with polymerization processes that employ either ultraviolet light or electron beams. One which stresses electron beam curing is U.S. Pat. No. 3,897,295, in which the composition subject to the electron beam includes an acrylate monomer selected from a particular specific group, and a homopolymer or copolymer of a substance or substances selected from the same group. The polymer is dissolved in the monomer and the monomer is ultimately polymerized to bind the adhesive together.
The disadvantage of using polymerization processes involving an electron beam, though, is that, generally, it is a rather indiscriminate polymerization process. In polymerization processes using an electron beam, the particulate bombardment of the polymerizable mixture can lead to chain scission of the developing polymer, resulting in an inability to control the molecular weight of the polymer and the crosslink density into the most desired range.
In order to avoid the above-discussed disadvantages incurred with the use of an electron beam, some have chosen to use a one step low-intensity (for example, 0.1 to 7 mW/cm.sup.2) UV photopolymerization process. See, for example, U.S. Pat. No. 4,181,752. Whereas the use of relatively low intensity UV light is very desirable for building higher molecular weight acrylic PSA's with good performance properties, the use of low intensity UV light leads to low manufacturing rates, so an increase in the speed of the photopolymerization process would be desirable. However, if one attempts to increase the speed of the low intensity UV light-based process by increasing the amount of the photoinitiator employed (for example, benzoin ethers, benzil ketals, etc.), undesirable lower molecular weight polymers will be obtained.
Furthermore, for thick adhesives an uneven polymerization from the front surface to the back surface of an irradiated adhesive composition occurs due to absorption of the UV radiation by the polymerizable mixture through the thickness of the coating. This results in a gradient in the conversion, molecular weight and distribution through the thickness of the cured material, which can lead to inferior performance of the final PSA product. In addition to the above discussed considerations, UV light-based processes generally require rigorous exclusion of oxygen during the polymerization process and are limited to essentially non-volatile acrylic monomers and to constructions that are substantially transparent to UV irradiation. Furthermore, controlling the polymerization reaction exotherm is still necessary.
A number of modifications and variations of the UV light-based processes are known. (See for example, U.S. Pat. Nos. 4,415,615 and 4,513,039). For example, a pressure sensitive adhesive composition is prepared by coating the polymerization mixture onto a web and polymerizing via UV irradiation, wherein the polymerization step is carried out in an inert atmosphere (Japanese Kokai No. HEI 5-5014). Alternatively, the UV light polymerization step is carried out while the coated web is immersed in water Japanese Kokai. No. HEI 4-41576).
In view of the foregoing discussed disadvantages and limitations that exist with the use of conventional polymerization processes, improvements are continuously desired and sought by those within the industry. It was against this background that an improved polymerization process for producing adhesives, and in particular acrylic-based adhesives and tapes was sought.